The present invention relates to a slant short-period grating used as an optical filter or the like in the field of optical communications and the like.
This application is based on Patent Application No. 2000-183796 filed in Japan, the contents of which are incorporated herein by reference.
An optical fiber grating is one example of a fiber type of optical filter. There are long-period grating (LPG) and short-period grating (SPG) types of optical fiber grating.
Conventionally, an optical fiber grating is formed by changing the refractive index of a core at a predetermined grating period in the longitudinal direction of the core. Note that the term grating period refers to the period of this refractive index change. An LPG grating period is approximately several hundred xcexcm.
In LPG, in a grating section in which a change in the refractive index is formed, light of a predetermined wavelength region from the incident light is coupled with a forward clad mode that moves forward in the same direction as the incident light, and transmitted light is obtained from which the light of this wavelength region has been lost.
In contrast to this, in SPG, the grating period is approximately one half to one third the wavelength of the light. Namely, if the operating wavelength is in the vicinity of 1.55 xcexcm, then a value of, for example, approximately one third of this is set. As a result, the light of a predetermined wavelength region, from among waveguide modes that are propagated along the core of an optical fiber, is reflected and coupled with a reflection mode, and transmitted light is obtained from which this light has been lost.
LPG has the advantage that there are no micro ripples that cause degradation in the signal waveform. The term micro ripples refers to minute fluctuations in the wavelength spectrum of transmitted light when the horizontal axis is the wavelength and the vertical axis is the transmittance. Therefore, LPG enables a smooth characteristic to be obtained in the wavelength spectrum. A further advantage is that reflected light is practically non-existent.
However, LPG has the disadvantage of it being difficult to obtain arbitrary transmission characteristics due to the difficulty of adjusting the transmission characteristics.
In SPG, in addition to the amount of change in the refractive index of the grating portion and the grating period, by employing a chirped grating in which the grating period is changed by being gradually extended or shortened in the longitudinal direction thereof, it is possible to widen the wavelength region of the lost light and to adjust the intensity of the lost light, and it is possible to obtain arbitrary transmission characteristics comparatively freely.
However, in SPG, multiple reflections are generated by the action of reflected light, and as a result, micro ripples are generated in the wavelength spectrum of the transmitted light creating the problem of it not being possible to obtain smooth wavelength spectrum characteristics. There is also the problem that there is a large amount of reflected light.
For these reasons, recently, the freedom of design allowed by SPG has been used to further the development of slant SPG in which it is even more difficult for micro ripples to occur.
FIG. 24 is a side cross-sectional view showing an example of a slant SPG. A description will now be given of the production method for this slant SPG
The symbol 1 in the drawing is a core. An optical fiber is formed by providing a clad 2 having a lower refractive index than the core 1 on the outer periphery of the core 1.
The core 1 and clad 2 are both formed from quartz glass. A photosensitive dopant that raises the refractive index of quartz glass when light of a specific wavelength is irradiated onto it is doped to the core 1. Normally, germanium is used as the photosensitive dopant. The refractive index is raised when ultraviolet light of approximately 240 nm is irradiated onto the germanium doped quartz glass.
Accordingly, when light is irradiated at a predetermined grating period in the longitudinal direction of the core 1 from one side surface of the optical fiber by the interposition of a phase mask or the like, the refractive index of that portion of the core 1 receiving the irradiated light is raised so that a grating portion 4 in which a plurality of high refractive index portions 3, 3, . . . are arranged at a predetermined grating period is obtained.
The high refractive index portions 3, 3, . . . are formed on an inclination so as to cut across the core 1 without being orthogonal to the center axis B of the core 1. Moreover, a plurality of high refractive index portions 3, 3, . . . are arranged parallel to each other in the longitudinal direction of the core 1. The direction of a line A that is perpendicular to a high refractive index portions 3 is known as the grating direction. Alternatively, this direction is known as the lattice vector direction of the grating portion.
An angle xcex8 between the grating direction A and the center axis B of the core 1 is known as the slant angle. The size of the inclination of the refractive index portions 3 is represented by this angle xcex8. Note that in normal SPG the grating direction matches the center axis of the core 1 so that the angle xcex8 is zero.
As a result, light that moves along the core 1 in the same direction as that of the incident light and whose waveguide mode is reflected by the grating portion 4 is irradiated onto the clad 2, and couples with a reverse clad mode that is moving in the opposite direction to the incident light. Namely, because it does not couple with a reflection mode that moves in reverse along the core 1, it is difficult for multiple reflections to occur. It is therefore possible to reduce the intensity of the micro ripples obtained in the wavelength spectrum.
FIGS. 25A, 25B, 26A, and 26B show wavelength spectrums for various slant angles.
Because the waveguide mode couples with a plurality of reverse clad modes, in the waveguide spectrum a plurality of loss peaks are aligned adjacent to each other.
If the slant angle is increased from 0 degrees to 2.9 degrees, 4 degrees, and 5.8 degrees, then the coupling with the reflection mode of the waveguide modes is smallest at 4 degrees. When the slant angle is further increased to 5.8 degrees, then the coupling begins to increase again. Namely, periodic characteristics are demonstrated in which couplings with reflection modes repeatedly increase and decrease as the slant angle increases.
The angle at which the couplings with reflection modes first reach the minimum value is known as the reflection suppression angle (4 degrees in this example, as is shown in FIG. 26A).
In slant SPG if the slant angle is set in the vicinity of the reflection suppression angle, it is possible to reduce the effects of micro ripples.
However, in a slant SPG that uses a typical single mode optical fiber that has a core and clad having a lower refractive index than the core provided on the outer periphery of the core, with the core being formed from germanium doped quartz glass while the clad is formed from pure quartz glass, if the slant angle is set in the vicinity of the reflection suppression angle, the region where the waveguide mode couples with a clad mode is extended, creating the drawback that it is not possible to obtain a steep wavelength spectrum.
FIG. 27 shows an example of a wavelength spectrum of slant SPG transmitted light obtained when, in the core of the above type of typical single mode optical fiber, the slant short period grating portion is formed such that the slant angle is in the vicinity of the reflection suppression angle at a fixed grating period. The loss region (the region of peak loss) reaches as far as 20 nm or more.
Furthermore, in a slant SPG it is possible in some cases to make divisions into main bands, which are wavelength regions where a large loss peak is obtained in the transmitted light wavelength spectrum, and side bands, which are small wavelength regions that appear on the short wavelength side of the main bands. There are also cases in which unnecessary ghost mode peaks are present in portions of the long wavelength side of the main band loss peaks, and cases in which there is a large side band transmission loss that becomes noise that appears to be parallel to the main band loss peaks.
If ghost peaks are present, or if there is a large side band transmission loss, then essentially it is not possible to make the loss band sufficiently narrow, and, in some cases, a steep wavelength spectrum cannot be obtained.
Moreover, even if the same light exposure, namely, the same change in the refractive index is provided, if the area of the main band of the transmission loss (referred to below on occasion as xe2x80x9ctransmission loss areaxe2x80x9d) is small, it is necessary to lengthen the exposure in order to obtain the same filter characteristics. This becomes a drawback during production.
Thus, in slant SPG, various problems exist such as difficulty of obtaining a steep wavelength spectrum, reducing the ghost mode peaks, reducing side band transmission loss, and increasing the transmission loss area. Accordingly, in some cases, it is difficult to obtain the desired characteristics, and there is still an insufficient degree of freedom allowed when designing optical characteristics. In particular, it has often been difficult to obtain a narrow loss band.
Slant SPG is used, for example, to equalize the wavelengthxe2x80x94gain characteristics of erbium (Er) doped optical fiber amplifiers. Slant SPG that allows a variety of designs for dealing with the optical characteristics of the gainxe2x80x94wavelength characteristics of these Er doped optical fiber amplifiers is desirable. When slant SPG is used for the Er doped optical fiber amplifiers, it is preferable for the ghost modes and the side band transmission loss of the slant SPG not to generate problems.
The present invention was conceived in view of the above circumstances and it is one object of the present invention is to provide a slant SPG that enables the free designing of optical characteristics.
Specifically, one object of the present invention is to provide a slant SPG having a narrow loss band in a transmitted light wavelength spectrum. In addition, one object of the present invention is to provide a slant SPG that has a greater transmission loss area using the same refractive index change.
In addition, one object of the present invention is to provide a slant SPG in which ghost mode peaks are decreased. In addition, one object of the present invention is to provide a slant SPG that enables a reduction in side band transmission loss to be achieved.
In order to achieve the above aims, in the present invention the inventions described below are proposed.
The first aspect of the present invention is a slant short-period grating which is obtainable by irradiating light onto an optical fiber having a core and a clad provided on an outer periphery of the core, the core being formed from quartz glass to which has been doped a photosensitive dopant that changes a refractive index of the quartz glass by light irradiation, and the clad having one or two or more layers with at least the layer that is adjacent to the core being a photosensitive layer formed from quartz glass to which has been doped a photosensitive dopant that changes a refractive index of the quartz glass by light irradiation, and thereby a grating portion is formed by changing the refractive index of the photosensitive layer of the clad and the core at a predetermined grating period along a longitudinal direction of the optical fiber by a predetermined slant angle,
wherein a diameter of the core is 5 xcexcm or more;
wherein a relative photosensitivity ratio of the core relative to the photosensitive layer of the clad that is adjacent to the core satisfies Formula (1) below:
0.2xe2x88x920.1xc2x7(Vxe2x88x921.7)xe2x89xa6Pxe2x89xa60.1a{0.41xe2x88x920.33xc2x7(Vxe2x88x921.7)}xe2x80x83xe2x80x83(1)
in Formula (1), a is the diameter of the core in units of xcexcm, V is a standardized frequency, and P is the relative photosensitivity ratio of the core relative to the photosensitive layer of the clad that is adjacent to the core; and
wherein the slant angle is set to such an angle that loss due to coupling of a waveguide mode with a reflection mode is minimum.
The second aspect of the present invention is a slant short-period grating, in the slant short-period grating according to first aspect of the invention, wherein the diameter of the core is 7 xcexcm or more.
The third aspect of the present invention is a slant short-period grating, in the slant short-period grating according to first aspect of the invention, wherein the relative photosensitive ratio of the core is 0.1 to 0.4.
The fourth aspect of the present invention is a slant short-period grating, in the slant short-period grating according to first aspect of the invention, wherein an outer diameter of the photosensitive layer of the clad is four times or more the diameter of the core.
The fifth aspect of the present invention is a slant short-period grating, in the slant short-period grating according to first aspect of the invention, wherein the diameter of the core is 12 xcexcm or less.
The sixth aspect of the present invention is a slant short-period grating, in the slant short-period grating according to first aspect of the invention, wherein a comparative refractive index difference between the core and the clad is 0.5% or less.
The seventh aspect of the present invention is a slant short-period grating, in the slant short-period grating according to first aspect of the invention, wherein aluminum or phosphorous is doped to the core.
The eighth aspect of the present invention is a slant short-period grating which is obtainable by irradiating light onto an optical fiber having a core and a clad provided on an outer periphery of the core, the clad having one or two or more layers with at least one layer being a photosensitive layer formed from quartz glass to which has been doped a photosensitive dopant that changes a refractive index of the quartz glass by light irradiation, and thereby a grating portion is formed by changing the refractive index of the photosensitive layer at a predetermined grating period along a longitudinal direction of the optical fiber by a predetermined slant angle,
wherein a relative photosensitivity ratio of the core relative to the photosensitive layer of the clad that has the highest photosensitivity satisfies Formula (2) below:
Pxe2x89xa6m1(Vxe2x88x922)+m2
m1=0.0041667a4xe2x88x920.13519a3+1.6206a2xe2x88x928.511a+16.291xe2x80x83xe2x80x83(2)
m2=xe2x88x920.00832827a2+0.18344axe2x88x920.6912
however, when P equals 0 or smaller or is imaginary number, P is 0
in Formula (2), a is the diameter of the core in units of xcexcm, V is a standardized frequency, and P is the relative photosensitivity ratio of the core relative to the photosensitive layer of the clad that has the highest photosensitivity.
The ninth aspect of the present invention is a slant short-period grating which is obtainable by irradiating light onto an optical fiber having a core and a clad provided on an outer periphery of the core, the clad having one or two or more layers with at least one layer being a photosensitive layer formed from quartz glass to which has been doped a photosensitive dopant that changes a refractive index of the quartz glass by light irradiation, and thereby a grating portion is formed by changing the refractive index of the photosensitive layer at a predetermined grating period along a longitudinal direction of the optical fiber by a predetermined slant angle,
wherein a relative photosensitivity ratio of the core relative to the photosensitive layer of the clad that has the highest photosensitivity satisfies Formula (3) below:
Pxe2x89xa7(Vxe2x88x921.7868)0.048522+0.17416Vxe2x88x921.121xe2x80x83xe2x80x83(3)
however, when P equals 0 or smaller or is imaginary number, P is 0
in Formula (3), a is the diameter of the core in units of xcexcm, V is a standardized frequency, and P is the relative photosensitivity ratio of the core relative to the photosensitive layer of the clad that has the highest photosensitivity.
The tenth aspect of the present invention is a slant short-period grating which is obtainable by irradiating light onto an optical fiber having a core and a clad provided on an outer periphery of the core, the clad having one or two or more layers with at least one layer being a photosensitive layer formed from quartz glass to which has been doped a photosensitive dopant that changes a refractive index of the quartz glass by light irradiation, and thereby a grating portion is formed by changing the refractive index of the photosensitive layer at a predetermined grating period along a longitudinal direction of the optical fiber by a predetermined slant angle,
wherein a relative photosensitivity ratio of the core relative to the photosensitive layer of the clad that has the highest photosensitivity satisfies Formula (4) below:
Pxe2x89xa7m1(axe2x88x92m2)m3
m1=xe2x88x920.28947+0.17702V
m2=xe2x88x92344.28+543.53Vxe2x88x92272.8V2+44.494V3xe2x80x83xe2x80x83(4)
m3=0.96687xe2x88x920.24791V
however, when P equals 0 or smaller or is imaginary number, P is 0
in Formula (4), a is the diameter of the core in units of xcexcm, V is a standardized frequency, and P is the relative photosensitivity ratio of the core relative to the photosensitive layer of the clad that has the highest photosensitivity.
The eleventh aspect of the present invention is a slant short-period grating, in the slant short-period grating according to any of the eighth to tenth aspects of the invention, wherein the slant angle is set such that loss due to coupling of a waveguide mode with a reflection mode is minimum.
The twelfth aspect of the present invention is a slant short-period grating, in the slant short-period grating according to any of the eighth to tenth aspects of the invention, wherein the relative photosensitivity ratio of the core relative to the photosensitive layer is 0.2 or more.
The thirteenth aspect of the present invention is a slant short-period grating, in the slant short-period grating according to any of the eighth to tenth aspects of the invention, wherein the grating period is a chirped pitch, and the chirping ratio of the grating period is 20 nm/cm or less.
The fourteenth aspect of the present invention is a slant short-period grating, in the slant short-period grating according to any of the first to tenth aspects of the invention, wherein a bend loss of an optical fiber under conditions of a wavelength of 1,550 nm and a winding diameter of 60 mm, is 1 dB/m or less.
The fifteenth aspect of the present invention is a slant short-period grating, in the slant short-period grating according to any of the first to tenth aspects of the invention, wherein a bend loss of an optical fiber in conditions of a wavelength of 1,550 nm and a winding diameter of 40 mm is 0.1 dB/m or less.
The sixteenth aspect of the present invention is a slant short-period grating, in the slant short-period grating according to any of the first to tenth aspects of the invention, wherein a mode field diameter of a waveguide mode of the optical fiber in an operating wavelength of the slant short-period fiber grating is 15 xcexcm or less.
The seventeenth aspect of the present invention is a slant short-period grating, in the slant short-period grating according to any of the first to tenth aspects of the invention, wherein the outer diameter of the photosensitive layer is 1.5 times or more the size of the mode field diameter of a waveguide mode of the optical fiber in an operating wavelength of the slant short-period fiber grating
The eighteenth aspect of the present invention is a slant short-period grating, in the slant short-period grating according to any of the first to tenth aspects of the invention, wherein the outer diameter of the photosensitive layer is 60 xcexcm or less.
The nineteenth aspect of the present invention is a slant short-period grating, in the slant short-period grating according to any of the first to tenth aspects of the invention, wherein the length of the grating portion is 1 to 100 mm.
The twentieth aspect of the present invention is an optical amplifier module comprising the slant short-period grating according to any of the first to tenth aspects and an optical amplifier, wherein gain equalization of the optical amplifier is performed by the slant short-period grating.
The twenty-first aspect of the present invention is an optical amplifier module, in the optical amplifier module according to the twentieth aspect, wherein the optical amplifier is an erbium doped optical fiber amplifier.
The twenty-second aspect of the present invention is an optical communication system that employs the optical amplifier module according to the twentieth aspect.
The twenty-third aspect of the present invention is a manufacturing method for a slant short-period grating in which a slant short-period grating is designed and manufactured such that the conditions described in any of the first to tenth aspects are satisfied.